# std::ranges::set_intersection, std::ranges::set_intersection_result

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::input_iterator I1, std::sentinel_for<I1> S1, std::input_iterator I2, std::sentinel_for<I2> S2, std::weakly_incrementable O, class Comp = ranges::less, class Proj1 = std::identity, class Proj2 = std::identity > requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2> constexpr set_intersection_result<I1, I2, O> set_intersection( I1 first1, S1 last1, I2 first2, S2 last2, O result, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); | (1) | (since C++20)
template< ranges::input_range R1, ranges::input_range R2, std::weakly_incrementable O, class Comp = ranges::less, class Proj1 = std::identity, class Proj2 = std::identity > requires std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>, O, Comp, Proj1, Proj2> constexpr set_intersection_result<ranges::borrowed_iterator_t<R1>, ranges::borrowed_iterator_t<R2>, O> set_intersection( R1&& r1, R2&& r2, O result, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); | (2) | (since C++20)
Helper types |  |
template< class I1, class I2, class O > using set_intersection_result = ranges::in_in_out_result<I1, I2, O>; | (3) | (since C++20)

Constructs a sorted range beginning at result consisting of elements that are found in both sorted input ranges `[`first1`,` last1`)` and `[`first2`,` last2`)`. If some element is found `m` times in `[`first1`,` last1`)` and `n` times in `[`first2`,` last2`)`, the first min(m, n) elements will be copied from the first range to result. The order of equivalent elements is preserved.

The behavior is undefined if

- the input ranges are not sorted with respect to comp and proj1 or proj2, respectively, or
- the resulting range overlaps with either of the input ranges.

1) Elements are compared using the given binary comparison function comp.
2) Same as (1), but uses r1 as the first range and r2 as the second range, as if using ranges::begin(r1) as first1, ranges::end(r1) as last1, ranges::begin(r2) as first2, and ranges::end(r2) as last2.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first1, last1 | - | iterator-sentinel pair denoting the first input sorted range
first2, last2 | - | iterator-sentinel pair denoting the second input sorted range
r1 | - | the first sorted input range
r2 | - | the second sorted input range
result | - | the beginning of the output range
comp | - | comparison to apply to the projected elements
proj1 | - | projection to apply to the elements in the first range
proj2 | - | projection to apply to the elements in the second range

### Return value

{last1, last2, result_last}, where result_last is the end of the constructed range.

### Complexity

At most \(\scriptsize 2\cdot(N_1+N_2)-1\)2·(N1+N2)-1 comparisons and applications of each projection, where \(\scriptsize N_1\)N1 and \(\scriptsize N_2\)N2 are ranges::distance(first1, last1) and ranges::distance(first2, last2), respectively.

### Possible implementation

    struct set_intersection_fn
    {
        template<std::input_iterator I1, std::sentinel_for<I1> S1,
                 std::input_iterator I2, std::sentinel_for<I2> S2,
                 std::weakly_incrementable O, class Comp = ranges::less,
                 class Proj1 = std::identity, class Proj2 = std::identity>
        requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
        constexpr ranges::set_intersection_result<I1, I2, O>
            operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                       O result, Comp comp = {},
                       Proj1 proj1 = {}, Proj2 proj2 = {}) const
        {
            while (!(first1 == last1 or first2 == last2))
            {
                if (std::invoke(comp, std::invoke(proj1, *first1),
                                      std::invoke(proj2, *first2)))
                    ++first1;
                else if (std::invoke(comp, std::invoke(proj2, *first2),
                                           std::invoke(proj1, *first1)))
                    ++first2;
                else
                    *result = *first1, ++first1, ++first2, ++result;
            }
            return {ranges::next(std::move(first1), std::move(last1)),
                    ranges::next(std::move(first2), std::move(last2)),
                    std::move(result)};
        }

        template<ranges::input_range R1, ranges::input_range R2,
                 std::weakly_incrementable O, class Comp = ranges::less,
                 class Proj1 = std::identity, class Proj2 = std::identity>
        requires std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
                                O, Comp, Proj1, Proj2>
        constexpr ranges::set_intersection_result<ranges::borrowed_iterator_t<R1>,
                                                  ranges::borrowed_iterator_t<R2>, O>
            operator()(R1&& r1, R2&& r2, O result, Comp comp = {},
                       Proj1 proj1 = {}, Proj2 proj2 = {}) const
        {
            return (*this)(ranges::begin(r1), ranges::end(r1),
                           ranges::begin(r2), ranges::end(r2),
                           std::move(result), std::move(comp),
                           std::move(proj1), std::move(proj2));
        }
    };

    inline constexpr set_intersection_fn set_intersection {};

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <vector>

    void print(const auto& v, const auto& rem)
    {
        std::cout << "{ ";
        for (const auto& e : v)
            std::cout << e << ' ';
        std::cout << '}' << rem;
    }

    int main()
    {
        const auto in1 = {1, 2, 2, 3, 4, 5, 6};
        const auto in2 = {2, 2, 3, 3, 5, 7};
        std::vector<int> out {};

        std::ranges::set_intersection(in1, in2, std::back_inserter(out));

        print(in1, " ∩ "), print(in2, " = "), print(out, "\n");
    }

Output:

    { 1 2 2 3 4 5 6 } ∩ { 2 2 3 3 5 7 } = { 2 2 3 5 }

### See also

ranges::set_union (C++20) | computes the union of two sets (algorithm function object)
ranges::set_difference (C++20) | computes the difference between two sets (algorithm function object)
ranges::set_symmetric_difference (C++20) | computes the symmetric difference between two sets (algorithm function object)
ranges::includes (C++20) | returns true if one sequence is a subsequence of another (algorithm function object)
set_intersection | computes the intersection of two sets (function template)

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